1. Field of the Invention
The present invention is directed to a method for operating a computed tomography apparatus, and to a computed tomography apparatus, of the type for conducting a spiral scan of an examination subject.
2. Description of the Prior Art
Methods for operating a computed tomography apparatus are known wherein, for scanning a subject with a conical ray beam emanating from a focus which strikes with a matrix-like detector array for detecting the radiation, the focus is moved relative to the subject on a spiral path around a system axis. The detector array supplies output data corresponding to the incident radiation, and images relative to the system axis are reconstructed from the output data respectively supplied during the movement of the focus has each spiral segment. Computed tomography (CT) systems are known having a radiation source with a focus from which a conical ray beam emanates, a matrix-like detector array for detecting the radiation, the detector array supplying output data corresponding to the incident radiation, means for generating a relative motion between the radiation source and the detector array, and the subject, and an image computer to which the output data are supplied. For scanning the subject with the ray beam and the two-dimensional detector array, motion relative to a system axis is generated such that the focus moves relative to the system axis on a helical spiral path whose center axis corresponds to the system axis. The image computer uses the output data respectively supplied during the motion of the focus in spiral segments to reconstruct images having an image plane inclined relative to the system axis.
Such a procedure is referred to as spiral-CT and such a method and CT apparatus are disclosed by U.S. Pat. No. 5,802,134.
The spiral path of the focus F illustrated in FIG. 1 is described by the following equations:
                                                                                          x                  f                                =                                ⁢                                                      -                                          R                      f                                                        ⁢                  cos                  ⁢                                                                          ⁢                  α                                                                                                                          y                  f                                =                                ⁢                                                      R                    f                                    ⁢                  sin                  ⁢                                                                          ⁢                  α                                                                                                                          z                  f                                =                                ⁢                                  S                  ·                  p                  ·                                      α                                          2                      ⁢                      π                                                                                                          ⁢                                  ⁢        and        ⁢                                  ⁢                              x            _                    =                      (                                                                                                      -                                              R                        f                                                              ⁢                    cos                    ⁢                                                                                  ⁢                    α                                                                                                                                          -                                              R                        f                                                              ⁢                    sin                    ⁢                                                                                  ⁢                    α                                                                                                                    Sp                    ⁢                                          α                                              2                        ⁢                        π                                                                                                                  )                                              (        1        )            
When the detector elements of the detector array are arranged in rows proceeding transversely relative to the system axis Z and in columns proceeding parallel to the system Z, S stands for the extent of a detector row in the direction of the system axis and p stands for the pitch, and
  p  =      h    S  applies, where h stands for the slope of the spiral path per revolution of the focus F. α is the projection angle, and an image plane associated with data that were registered over a projection angle range of ±α is considered below, with the reference projection belonging to the image plane being at αr=0, i.e. it represents the middle of the projection angle range ±α. Below, αr is referred to as the reference projection angle.
In conventional spiral-CT, tomograms referred to as transverse tomograms are reconstructed, i.e. images for image planes that reside at a right angle relative to the system axis z and that thus contain the x-axis and y-axis, the x-axis and y-axis being at a right angle relative to one another and to the system axis z.
In the aforementioned U.S. Pat. No. 5,802,134, in contrast, images are reconstructed for image planes that, according to FIG. 2, are inclined by an inclination angle γ around the x-axis relative to the system axis z. As a result, the advantage (at least theoretically) is achieved that the images contain fewer artifacts when the inclination angle γ is selected such that a good, optimum matching of the image plane to the spiral path is established insofar as possible according to a suitable error criterion, for example minimum square average of the spacing of all points of the spiral segment from the image plane measured in the z-direction.
In U.S. Pat. No. 5,802,134, fan data, i.e. data registered in known fan geometry, are employed for the reconstruction, the data having been acquired with the motion of the focus over a spiral segment having proceeding through a 180° plus the fan angle or cone angle, for example 240°. With respect to the reference projection angle αr=0, the following applies to the normal vector of the image plane:
            n      US        ⁡          (      γ      )        =            (                                    0                                                                              -                sin                            ⁢                                                          ⁢              γ                                                                          cos              ⁢                                                          ⁢              γ                                          )        .  The optimum inclination angle γ is obviously dependent on the slope of the spirals and thus on the pitch p.
Fundamentally, the method disclosed in U.S. Pat. No. 5,802,134 can be employed for arbitrary values of the pitch p. However, an optimum utilization of the detector area available and thus of the radiation dose applied to the patient for image acquisition (detector and thus dose utilization), is not possible below the maximum pitch Pmax. This is because even though a given transverse slice, i.e. a slice of the subject residing at a right angle relative to the system axis z, is scanned over a spiral segment that is longer than 180° plus the fan or cone angle, only a spiral segment having the length 180° plus the cone angle can be utilized for values of the pitch p below the maximum pitch Pmax, since the utilization of a longer spiral segment would make it impossible to adapt the image plane adequately well to the spiral path.